This article first describes a method of obtaining a chromium oxalate complex (NH4)3[Cr(C2O4)3] with extremely variable grain morphology (needles, polyhedra, platelets, spheres). The decomposition of these salts at 350-degrees-C in air leads to nonstoichiometric chromium oxides amorphous to X-rays, of composition CrO(x) where x is close to 1.9. The various oxides obtained at 350-degrees-C had very high specific surface areas of the order of several hundreds of m2 g-1 which varied considerably according to the grain morphology. They all possessed a significant mesopore area with pores centred around 5 nm. The crystallization of CrO1.9 into alpha-Cr2O3 occurred on raising the temperature above 350-degrees-C and was accompanied by an emission of oxygen which took place in two stages. The kinetics of the oxygen emission varied with the grain morphology and could be correlated with the differences in length of the oxygen diffusion path corresponding to each oxide form.